Process for the removal of highly concentrated carbon dioxide from high-pressure natural gas

ABSTRACT

This invention relates to a process for removing highly concentrated CO 2  from high-pressure natural gas and recovering it in a high-pressure state. 
     This process comprises the absorption step of bringing high-pressure natural gas having a CO 2  partial pressure of 2 kg/cm 2  or greater and a pressure of 30 kg/cm 2  or greater into gas-liquid contact with a regenerated CO 2  -lean absorbing fluid comprising a CO 2  absorbing fluid of which the difference in saturated CO 2  absorption level between 40° C. and 120° C. is not less than 30 Nm 3  per ton of solvent at a CO 2  partial pressure of 2 kg/cm 2 , whereby highly concentrated CO 2  present in the high-pressure natural gas is absorbed into the CO 2  -lean absorbing fluid to produce refined natural gas having a reduced CO 2  content and a CO 2  -rich absorbing fluid; and the regeneration step of heating the CO 2  -rich absorbing fluid without depressurizing it, whereby high-pressure CO 2  having a pressure of 10 kg/cm 2  or greater is liberated and a CO 2  -lean absorbing fluid is regenerated and recycled for use in the absorption step.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the removal of highlyconcentrated carbon dioxide (CO₂) from high-pressure natural gas. Moreparticularly, it relates to a process for the removal of highlyconcentrated CO₂ from high-pressure natural gas whereby, at a naturalgas production spot or the like, CO₂ is separated and removed from thecollected high-pressure natural gas having a pressure of 30 kg/cm² orgreater (the term "pressure" as used herein means an absolute pressure)to produce refined natural gas, and whereby the separated CO₂ can beobtained in a relatively high-pressure state which is beneficial for thepurpose of injection when it is utilized in tertiary oil recovery orstored permanently in an underground aquifer.

2. Description of the Related Art

It may usually happen that natural gas produced in a gas field containsan appreciable amount of CO₂. According to the necessity for reducingthe cost required to transport such natural gas from its production spotto a remote consumption place, and for adjusting its calorific value tothe standard at the consumption place, some CO₂ is previously removedtherefrom to produce refined natural gas having a CO₂ content rangingfrom 2-3 vol. % to ten-odd vol. %. Conventionally, the CO₂ separated byprimary refining at the natural gas production spot or in theneighborhood thereof has seldom been utilized. That is, such CO₂ hasbeen dumped directly into the atmosphere or has rarely been used as aninjection gas for tertiary oil recovery in an oil field. Accordingly,little consideration has been given to the pressure of the CO₂ separatedby the aforesaid refining process.

In recent years, global warming due to an increase of atmospheric CO₂has come to be regarded as a problem. Accordingly, the present situationis such that the CO₂ separated in the above-described manner must bepressurized in order to inject it into an underground aquifer for thepurpose of permanent storage or to use it positively for the purpose oftertiary oil recovery. However, in spite of the fact that high-pressurenatural gas is treated, the CO₂ separated by a conventionally employedprocess for the removal of CO₂ from natural gas has a low pressure closeto atmospheric pressure. This is disadvantageous in that, for theabove-described purpose of permanent storage or tertiary oil recovery,the CO₂ must be pressurized from a low pressure close to atmosphericpressure to a pressure of about 150 kg/cm² which is required forinjection.

SUMMARY OF THE INVENTION

As a result of intensive investigations on the above-described problemsconcerning the removal of CO₂ from natural gas and the disposal andutilization of the separated CO₂, the present inventors have found that,by employing a specific process using, among various absorbing fluidshaving the ability to absorb CO₂, an absorbing fluid having so-calledphysical absorbing power characterized by the marked temperaturedependence of saturated CO₂ absorption level, CO₂ having a much higherpressure than that obtained by conventional processes can be separatedat low energy cost with much more simplified equipment than used inconventional systems. The present invention has been completed on thebasis of this finding.

That is, the present invention provides a method by which carbon dioxidethat has conventionally been dumped into the atmosphere during thecollection of natural gas can be separated and recovered in ahigh-pressure state permitting underground dumping in conformance withrecent global environmental standards. This method enables the separatedcarbon dioxide to be dumped into the ground without using a compressor.Alternatively, it also enables the separated carbon dioxide to bereturned to the top of an underground oil stratum in an oil field. Thus,the intended object can be accomplished by employing a system of simpleconstruction.

According to the present invention, there is provided a process for theremoval of highly concentrated CO₂ from high-pressure natural gas whichcomprises the absorption step of bringing high-pressure natural gashaving a CO₂ partial pressure of 2 kg/cm² or greater and a pressure of30 kg/cm² or greater into gas-liquid contact with a regenerated CO₂-lean absorbing fluid comprising a CO₂ absorbing fluid of which thedifference in saturated CO₂ absorption level between 40° C. and 120+ C.is not less than 30 Nm³ per ton of solvent at a CO₂ partial pressure of2 kg/cm², whereby highly concentrated CO₂ present in the high-pressurenatural gas is absorbed into the CO₂ -lean absorbing fluid to producerefined natural gas having a reduced CO₂ content and a CO₂ -richabsorbing fluid; and the regeneration step of heating the CO₂ -richabsorbing fluid without depressurizing it, whereby high-pressure CO₂having a pressure of 10 kg/cm² or greater is liberated and a CO₂ -leanabsorbing fluid is regenerated and recycled for use in the absorptionstep.

After having absorbed CO₂, the CO₂ absorbing fluid used in the presentinvention is regenerated in the regeneration step where most of the CO₂is liberated therefrom, and then recycled for use in the absorptionstep. In the present invention, the CO₂ absorption capacity of theaforesaid CO₂ absorbing fluid must be such that the difference insaturated CO₂ absorption level between 40° C. and 120° C. is not lessthan 30 Nm³ per ton of solvent, preferably not less than 40 Nm³ per tonof solvent, at a CO₂ partial pressure of 2 kg/cm². Usually, if thetemperature and CO₂ partial pressure of a specific absorbing fluid aredetermined, the saturated CO₂ absorption level shows a definite valuebased on the saturated CO₂ absorption curve for the specific absorbingfluid, almost regardless of the type of the CO₂ -containing gas. In thepresent invention, CO₂ is removed from high-pressure natural gas havinga pressure of 30 kg/cm² or greater by absorbing it into a CO₂ -leanabsorbing fluid obtained by regenerating the CO₂ absorbing fluid in thesucceeding regeneration step, and the resulting CO₂ -rich absorbingfluid is heated without substantially depressurizing it, so as toliberate CO₂ therefrom. Accordingly, it is preferable to use anabsorbing fluid which can absorb CO₂ easily at a relatively lowtemperature and a lower CO₂ partial pressure than in the regenerationstep, and can liberate CO₂ easily when heated in the regeneration step,i.e., at a relatively high temperature and a relatively high CO₂ partialpressure.

The temperature dependence of the saturated CO₂ absorption level of anabsorbing fluid depends largely on the types of the chemical agent(s)and solvent constituting the absorbing fluid. In the present invention,the difference in saturated CO₂ absorption level between 40° C. and 120°C. at a certain CO₂ partial pressure (i.e., a CO₂ partial pressure of 2kg/cm²) is employed as an index to the ability of an absorbing fluid toabsorb CO₂ in the absorption step and liberate CO₂ when heated in theregeneration step. Thus, there is used an absorbing fluid of which thedifference in saturated CO₂ absorption level between 40° C. and 120° C.at a CO₂ partial pressure of 2 kg/cm² is not less than 30 Nm³ per ton ofsolvent and preferably not less than 40 Nm³ per ton of solvent. Noparticular limitation is placed on the type of the absorbing fluid,provided that its difference in saturated CO₂ absorption level between40° C. and 120° C. at the aforesaid CO₂ partial pressure is not lessthan 30 Nm³ per ton of solvent and it is stable at the heatingtemperature of the regeneration step. Moreover, when the CO₂ partialpressure is 2 kg/cm², the saturated CO₂ absorption level at 40° C.serves as an index to the CO₂ absorption capacity of the CO₂ -leanabsorbing fluid. In the present invention, it is preferable to use anabsorbing fluid of which this absorption level is not less than 30 Nm³per ton of solvent and more preferably not less than 40 Nm³ per ton ofsolvent.

Specific examples of the aforesaid absorbing fluid include an aqueoussolution of N-methyldiethanolamine (MDEA), an aqueous solution oftriethanolamine, and an aqueous solution of potassium carbonate, as wellas these solutions having a CO₂ absorption promoter (e.g., piperazine)added thereto.

When it is desired to absorb and remove highly concentrated CO₂ presentin natural gas and thereby obtain the separated CO₂ in a high-pressurestate suitable for use in tertiary oil recovery or for the undergroundstorage of CO₂, the process of the present invention has the advantageof simplifying the equipment and reducing the energy cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary system which can be employed to carryout the process for the removal of CO₂ from high-pressure natural gas inaccordance with the present invention; and

FIG. 2 illustrates an exemplary system which can be employed to carryout a conventional process for the removal of CO₂ from high-pressurenatural gas.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An exemplary system which can be employed to carry out the process ofthe present invention is specifically described below with reference toFIG. 1. For purposes of comparison, an exemplary system which hasconventionally been employed is illustrated in FIG. 2.

In FIG. 1, reference numeral 1 designates natural gas; 2, an absorptiontower; 3, refined natural gas; 4, a CO₂ -rich absorbing fluid; 5, a heatexchanger; 6, a heater; 7, a regeneration tower; 8, a CO₂ -leanabsorbing fluid; 9, a cooler; 10, an overhead condenser; 11, aseparating drum; 12, a compressor; and 13, high-pressure CO₂. By way ofexample, the conditions for removing CO₂ from natural gas by employingthe system of FIG. 1 and using a 45 wt. % aqueous solution of MDEA asabsorbing fluid are given below. Natural gas 1 having a CO₂ content of26 vol. %, a pressure of 58 kg/cm² and a temperature of 25° C. is fed tothe lower part of an absorption tower 2. This absorption tower 2 ispacked, for example, with an irregular packing material so that theascending natural gas will come into efficient gas-liquid contact with aCO₂ -lean absorbing fluid fed to the upper part thereof. The refinednatural gas 3 having been freed of CO₂ by contact with the CO₂ -leanabsorbing fluid, which now has a CO₂ content of 2 vol. %, a temperatureof 50° C. and a pressure of 58 kg/cm², is discharged from the top ofabsorption tower 2. On the other hand, the CO₂ -lean absorbing fluidhaving absorbed CO₂ turns into a CO₂ -rich absorbing fluid 4, which istransferred to a regeneration step by means of a pump and regeneratedtherein.

The regeneration step essentially comprises a heat exchanger 5, a heater6 and a regeneration tower 7. CO₂ -rich absorbing fluid 4 is heated inheat exchanger 5 by heat exchange with a hot CO₂ -lean absorbing fluid 8which will be described later, further heated with heater 6 using steamas a heat source, and then fed to regeneration tower 7. The purpose ofregeneration tower 7 is to separate CO₂, which has been liberated by theaforesaid heating, from the absorbing fluid to produce a CO₂ -leanabsorbing fluid. Although an additional heater such as a reboiler is notrequired, it may be installed as desired. The liberated CO₂ is cooled inoverhead condenser 10 and separated from any entrained absorbing fluidin a separating drum 11. This CO₂, which has a relatively high pressureof 55 kg/cm² at about 40° C., is compressed to a pressure of 150 kg/cm²by means of a compressor 12. The resulting high-pressure CO₂ is used forpurposes of tertiary oil recovery or stored in the earth.

On the other hand, the CO₂ -lean absorbing fluid 8 withdrawn from thebottom of regeneration tower 7, which has a temperature of about 140°C., is cooled in heat exchanger 5 by heating CO₂ -rich absorbing fluid 4as described above, further cooled with a cooler 9 using cooling water,seawater or the like, and then recycled to the upper part of absorptiontower 2.

Now, the above-described process for the removal of CO₂ fromhigh-pressure natural gas in accordance with the present invention iscompared with a conventional system illustrated in FIG. 2. In FIG. 2,the units and elements having the same functions as those shown in FIG.1 are designated by the same reference numerals. In the system of FIG.2, natural gas 1 is fed in the same manner as in FIG. 1, and CO₂ isabsorbed and removed therefrom under the same conditions as in FIG. 1.The resulting gas is discharged from the top of absorption tower 2 asrefined natural gas 3. On the other hand, the resulting CO₂ -richabsorbing fluid 4 is withdrawn by means of a pump and then flashed in afirst flash drum 21 to produce the liberated CO₂ 32 and the absorbingfluid having a reduced CO₂ content. The latter is further heated with aheater 22 and fed to a second flash drum 23 where it is flashed again.The liberated CO₂ 33 is recovered by way of an overhead condenser 24 anda first separating drum 25. Since the CO₂ liberated in second flash drum23 has approximately atmospheric pressure, it is compressed with a firstcompressor 26 and combined with the aforesaid liberated CO₂ 32. Thecombined CO₂ is passed through a second separating drum 27, a secondcompressor 28, a third separating drum 29 and a third compressor 30 toobtain high-pressure CO₂ 31 having a pressure of about 150 kg/cm². Onthe other hand, the CO₂ -lean absorbing fluid 8 obtained atapproximately atmospheric pressure is pressurized with a pressurizingpump and cooled with a cooler 34 to produce a CO₂ -lean absorbing fluid8', which is fed to the upper part of absorption tower 2.

It is evident from a comparison of FIGS. 1 and 2 that, in order toobtain the liberated CO₂ having an identical pressure of 150 kg/cm², theprocess of the present invention illustrated in FIG. 1 brings about amarked simplification of equipment. In particular, it can be seen thatthe number of compressors involving a rotary driving mechanism requiringtroublesome operational management can be largely decreased. Moreover,in the case where the aforesaid aqueous solution of MDEA is used asabsorbing fluid, the amounts of energy required for both systems havebeen calculated from the saturated CO₂ absorption curve for theabsorbing fluid, and the results thus obtained are summarized inTable 1. The thermal efficiency of power units is supposed to be 25%.

                  TABLE 1                                                         ______________________________________                                                                 FIG. 2                                                                 FIG. 1 (Comparative                                                           (Example)                                                                            Example)                                             ______________________________________                                        Feed rate of natural gas (Nm.sup.3 /H)                                                            37,852   37,852                                           (25° C.; 58 kg/cm.sup.2 ; CO.sub.2 content,                            26 vol. %)                                                                    Discharge rate of refined natural                                                                 28,310   28,310                                           gas (Nm.sup.3 /H) (50° C.; 57.9 kg/cm.sup.2 ;                          CO.sub.2 content, 2 vol. %)                                                   Circulation rate of absorbing                                                                     555      555                                              fluid (T/H)                                                                   Total flow rate of liberated CO.sub.2                                                             9,542    9,542                                            (Nm.sup.3 /H)                                                                 CO.sub.2 pressure at the outlet of                                                                --       4.8                                              first compressor (kg/cm.sup.2)                                                (Compressor power, kW)       (450)                                            CO.sub.2 pressure at the outlet of                                                                --       30                                               second compressor (kg/cm.sup.2)                                               (Compressor power, kW)       (937)                                            CO.sub.2 pressure at the outlet of                                                                --       150                                              third compressor (kg/cm.sup.2)                                                (Compressor power, kW)       (695)                                            CO.sub.2 pressure at the outlet of                                                                150      --                                               compressor 12 (kg/cm.sup.2)                                                   (Compressor power, kW)                                                                            (181)                                                     Power other than CO.sub.2 compressor                                                              80       1,159                                            power (kW)                                                                    Total power (kW)    261      3,241                                            Total quantity of heat used to                                                                    10.5 × 10.sup.6                                                                  1.07 × 10.sup.6                            heat CO.sub.2 -rich absorbing fluid                                           (Kcal/H)                                                                      Total power calculated by                                                                         3,313    3,552                                            converting the quantity of heat                                               into power (kW)                                                               (thermal efficiency η = 25%)                                              ______________________________________                                    

We claim:
 1. A process for a removal of highly concentrated carbondioxide from high-pressure natural gas which comprises:an absorptionstep of bringing high-pressure natural gas having a carbon dioxidepartial pressure of 2 kg/cm² (absolute pressure) or greater, and apressure of 30 kg/cm² (absolute pressure) or greater, into gas-liquidcontact with a regenerated carbon dioxide-lean absorbing fluid, saidcarbon dioxide-lean absorbing fluid having saturated carbon dioxideabsorption levels at 40° C. and 12020 C. which differ by at least 30 Nm³per metric ton of solvent at a carbon dioxide partial pressure of 2kg/cm² (absolute pressure), whereby highly concentrated carbon dioxidepresent in the high-pressure natural gas is absorbed into the carbondioxide-lean absorbing fluid, to produce refined natural gas having areduced carbon dioxide content and a carbon dioxide-rich absorbingfluid; a regeneration step of heating the carbon dioxide-rich absorbingfluid without depressurizing it under conditions effective to liberatehigh pressure carbon dioxide having a pressure of 10 kg/cm² (absolutepressure) or greater and a carbon dioxide-lean absorbing fluid isregenerated; and a recycling step of recycling said regenerated carbondioxide-lean absorbing fluid to said absorption step.
 2. A process fordisposal of highly concentrated carbon dioxide present in high-pressurenatural gas which comprises:an absorption step of bringing high-pressurenatural gas having a carbon dioxide partial pressure of 2 kg/cm²(absolute pressure) or greater, and a pressure of 30 kg/cm² (absolutepressure) or greater, into gas-liquid contact with a regenerated carbondioxide-lean absorbing fluid, said carbon dioxide-lean absorbing fluidhaving saturated carbon dioxide absorption levels at 40° C. and 120° C.which differ by at least 30 Nm³ per metric ton of solvent at a carbondioxide partial pressure of 2 kg/cm² (absolute pressure), whereby highlyconcentrated carbon dioxide present in the high-pressure natural gas isabsorbed into the carbon dioxide-lean absorbing fluid, to producerefined natural gas having a reduced carbon dioxide content and a carbondioxide-rich absorbing fluid; a regeneration step of heating the carbondioxide-rich absorbing fluid without depressurizing it under conditionseffective to liberate high-pressure carbon dioxide having a pressure of10 kg/cm² (absolute pressure) or greater and carbon dioxide is dumpedinto an underground zone, and a carbon dioxide-lean absorbing fluid isregenerated; and a recycling step of recycling said regenerated carbondioxide-lean absorbing fluid to said absorption step.
 3. A process for adisposal of highly concentrated carbon dioxide present in high-pressurenatural gas which comprises:an absorption step of bringing high-pressurenatural gas having a carbon dioxide partial pressure of 2 kg/cm²(absolute pressure) or greater, and a pressure of 30 kg/cm² (absolutepressure) or greater, into gas-liquid contact with a regenerated carbondioxide-lean absorbing fluid, said carbon dioxide-lean absorbing fluidhaving saturated carbon dioxide absorption levels at 40° C. and 120° C.which differ by at least 30 Nm³ per metric ton of solvent at a carbondioxide partial pressure of 2 kg/cm² (absolute pressure), whereby highlyconcentrated carbon dioxide present in the high-pressure natural gas isabsorbed into the carbon dioxide-lean absorbing fluids to producerefined natural gas having a reduced carbon dioxide content and a carbondioxide-rich absorbing fluid; a regeneration step of heating the carbondioxide-rich absorbing fluid without depressurizing it under conditionseffective to liberate high-pressure carbon dioxide having a pressure of10 kg/cm² (absolute pressure) or greater and carbon dioxide is returnedto a top of an underground oil stratum, and a carbon dioxide-leanabsorbing fluid is regenerated; and a recycling step of recycling saidregenerated carbon dioxide-lean absorbing fluid to said absorption step.4. A process for disposal of highly concentrated carbon dioxide presentin high-pressure natural gas according to claim 1, wherein said carbondioxide absorbing fluid is an aqueous solution of one or more compoundsselected from the group consisting of N-methyldiethanolamine,triethanolamine and potassium carbonate.
 5. A process for disposal ofhighly concentrated carbon dioxide present in high-pressure natural gasaccording to claim 2, wherein said carbon dioxide absorbing fluid is anaqueous solution of one or more compounds selected from the groupconsisting of N-methyldiethanolamine, triethanolamine and potassiumcarbonate.
 6. A process for disposal of highly concentrated carbondioxide present in high-pressure natural gas according to claim 3,wherein said carbon dioxide absorbing fluid is an aqueous solution ofone or more compounds selected from the group consisting ofN-methyldiethanolamine, triethanolamine and potassium carbonate.
 7. Aprocess for disposal of highly concentrated carbon dioxide present inhigh-pressure natural gas according to claim 4, wherein said aqueoussolution further includes piperazine therein.
 8. A process for disposalof highly concentrated carbon dioxide present in high-pressure naturalgas according to claim 5, wherein said aqueous solution further includespiperazine therein.
 9. A process for disposal of highly concentratedcarbon dioxide present in high-pressure natural gas according to claim6, wherein said aqueous solution further includes piperazine therein.